You shed some fatigue and recovered, your heart rate will go down as a result. Bike fit may have something to do with your FTP result as you may have some muscles engaged that you weren’t before and they may take some time to get up to speed.
Like has been said above. I’ve been riding for a dozen years. I’m stronger now than I was 1-2 years in. It takes time to build enough of a base to be able to to the added volume and intensity to keep up with the progressive overload. And even then, you will have phases in your training. Embrace the lower scores because you will come back stronger. My FTP from last year at my peak was 302. I was able to maintain it for probably 3ish training blocks. Then it got to be more than I could bear, took a hit back to 295 for some base work and then after a couple rounds of base, was back at 304 feeling better than i did at any time last summer. If you don’t allow for the steps back, your body will strain with the steps forward and you won’t progress.